Method for producing glass preform

ABSTRACT

A method for producing a glass preform by synthesizing a porous glass preform by a vapor-phase synthesizing method and heating said porous glass preform in a vacuum or reduced-pressure atmosphere so as to consolidate said porous glass preform, which comprises the steps of: a first step of degassing said porous glass preform to thereby remove gas adsorbed or contained therein; a second step of temporarily contracting said porous glass preform at a temperature higher than a temperature in the first step and lower than a consolidation temperature; and a third step of consolidating said porous glass preform at the consolidation temperature; the degassing of said first step is determined in accordance with the bulk density of said porous glass preform.

FIELD OF THE INVENTION

The present invention relates to a method for producing a glass preform,and particularly, to a method for producing a low-loss optical fiberadapted for long distance transmission, and an apparatus for carryingout the method.

BACKGROUND OF THE INVENTION

A porous glass preform synthesized by a vapor-phase synthesizing methodsuch as a vapor-phase axial deposition method, (VAD method) or anoutside vapor deposition (OVD method), is subjected to high-temperatureheating treatment in an electric furnace consolidating it into a glasspreform. Conventionally, a porous glass preform is consolidated to betransparent by several methods, such as a zone heating method and auniform heating method. In the former method, a transversely shapedporous glass preform is consolidated by passing it through a narrowheating zone under ordinary pressure in an atmosphere of He or Hecontaining a small amount of halogen gas (especially, chlorine). In thelatter method, a porous glass preform is put into an electric furnacehaving a wide heating range and the temperature in the furnace is raisedgradually so that the whole length of the porous glass preform is heatedevenly in the atmosphere similar to that used in the former method.

For example, in a method for consolidating a porous glass preform,JP-A-62-176936 (The term "JP-A" used herein means an unexamined Japanesepatent application) discloses a method in which air, a hydroxyl groupand chlorine in the porous glass preform are removed stably with goodreproductivity by degassing while adjusting the quantity of ofatmospheric gas introduced into a furnace and the quantity of theatmospheric gas exhausted from the furnace, to thereby maintain aconstant pressure of the inside of the furnace. JP-A-5-24854 discloses amethod in which, in order to obtain a high-quality glass preform smallin variation of its outer diameter and little in residual air bubblesupon production of a transparent glass preform by heat treating a porousglass preform in a vacuum or reduced-pressure atmosphere, at least threeheat treating steps are conducted: a first step of heat treating theporous glass preform at a temperature so that the porous glass preformis not contracted; a second step of heat treating at a temperature whichis higher than the heat treating temperature in the first step and atwhich the porous glass preform is not consolidated, and a third step ofheat treating the porous glass preform at a temperature at which theporous glass preform is consolidated.

When a porous glass preform is degassed and consolidated to betransparent in a vacuum consolidation furnace, gas is apt to remain inthe porous glass preform at the time of degassing if the bulk density ofthe porous glass preform is high. Main components of the gas are water,air, hydrochloric acid, etc. Because the gas fills the gaps between theglass particles in the porous glass preform or is adsorbed on the glassparticles, the higher the bulk density is, the more the passage of thegas is clogged greatly so that the gas is hardly drawn out of the porousglass preform.

Such a porous glass preform may cause deterioration in strength andtransmission characteristic and, in addition, if heating is furthercontinued to consolidate the glass preform, there is a concern that thegas in the glass preform may expand to cause deformation and explosionof the glass preform. When the degassing time was increased by threetimes as much as the conventional time, there arose a problem that theproductivity was lowered largely, while the degassing per se was carriedout sufficiently.

SUMMARY OF THE INVENTION

The present invention is to solve the aforementioned problem.

Accordingly, it is an object of the invention to provide a method forproducing a high-quality glass preform efficiently by determining thedegassing time in accordance with the bulk density of a porous glasspreform used.

Other objects and effects of the present invention will become apparentfrom the following description.

The foregoing objects can be achieved by the following inventions.

(1) A method for producing a glass preform by synthesizing a porousglass preform by a vapor-phase synthesizing method and heating saidporous glass preform in a vacuum or reduced-pressure atmosphere so as toconsolidate said porous glass preform, which comprises the steps of:

a first step of degassing said porous glass preform to thereby removegas adsorbed or contained therein;

a second step of temporarily contracting said porous glass preform at atemperature higher than a temperature in the first step and lower than aconsolidation temperature; and

a third step of consolidating said porous glass preform at theconsolidation temperature,

wherein said first step is carried out while the degassing time isdetermined in accordance with the bulk density of said porous glasspreform.

(2) The method for producing a glass preform according to the aboveaspect (1), wherein said degassing of said porous glass preform iscarried out over a period of time determined by the following relationalexpression between a degassing time and the bulk density of a porousglass preform:

    Degassing Time (minute)=A×Bulk Density (g/cm.sup.3)

wherein A is from 201 to 340.

(3) The method for producing a glass preform according to the aboveaspect (2), wherein said degassing is carried out at a temperature offrom 900 to 1350° C.

(4) The method for producing a glass preform according to any one of theabove aspects (1) to (3), wherein the bulk density of said porous glasspreform is not lower than 0.6 g/cm³ or greater than 0.8 g/cm³.

(5) The method for producing a glass preform according to any one of theabove aspects (1) to (4), carried out by using an apparatus comprising:

a furnace body having a port opening formed in an upper portion thereoffor taking in and taking out a preform;

a heater disposed in said furnace body;

a furnace core pipe for isolating said heater and said preform from eachother;

an upper cover for sealing said port opening after insertion of saidpreform;

a radiation thermometer for monitoring a temperature of said furnacecore pipe; and

an exhaust pump for reducing pressure in said furnace body.

(6) An apparatus for producing a glass preform for carrying out a methodaccording to any one of the above aspects (1) to (4), which comprises:

a furnace body having a port opening formed in an upper portion thereoffor taking in and taking out a preform;

a heater disposed in said furnace body;

a furnace core pipe for isolating said heater and said preform from eachother;

an upper cover for sealing said port opening after insertion of saidpreform;

a radiation thermometer for monitoring a temperature of said furnacecore pipe; and

an exhaust pump for reducing pressure in said furnace body.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a conceptual view showing a vacuum consolidation furnaceadapted for carrying out the method of the present invention.

FIG. 2 is a graph showing the relationship between a degassing time andthe bulk density of a porous glass preform in relation to the feature ofthe method of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

In the method shown in the above aspect (1), increase in degassingtemperature at the first step may bring about acceleration of degassingso long as the degassing temperature is in a certain temperature range.If the degassing temperature exceeds the certain temperature range, theporous glass preform is contracted so that, on the contrary, thedegassing does not proceed. The suitable temperature range is from 900to 1350° C., preferably from 1200 to 1300° C. as described in the aboveitem (3). Accordingly, in degassing the porous glass preform, it isnecessary to change the degassing time within the aforementionedtemperature range depending on the bulk density of the porous glasspreform. The terminology "temporary contraction" means a state beforethe porous glass preform is consolidated, that is, a state in which theporous glass preform is made dense but not consolidated yet.

The porous glass preform used herein may be prepared by any one of a VADmethod, an OVD method, a sol-gel method, etc. Alternatively, a porousglass preform prepared by molding or pressure-molding glass fineparticles may be used. The term "porous glass preform" as used hereinincludes a composite glass preform in which a porous glass preform issynthesized on the outer circumference of a starting glass rod. In thiscase, the terminology "bulk density" means the weight per unit volume ofthe porous glass preform excluding the starting glass rod (i.e.: g/cm³).

Preferred Embodiment of the Invention

A starting glass rod is prepared, and porous glass is synthesized on theouter circumference of the rod by a VAD method to thereby produce acomposite glass preform having a bulk density not lower than 0.6 g/cm³,preferably from 0.6 to 0.8 g/cm³. The composite glass preform isconsolidated in a vacuum consolidation furnace configured according tothe present invention. As shown in FIG. 1, the vacuum consolidationfurnace 2 has a muffle tube 3, a heater 4 surrounding the furnace corepipe 3, an inert gas supply unit 5, inert gas flow meters 6 and 7, ports8 and 9 for supplying gas into the furnace, suction pumps 10 and 11 forcirculating inert gas for keeping the furnace in a vacuum orreduced-pressure atmosphere, pipings 12 and 13 for exhausting gas fromthe furnace body and the furnace core pipe, a seed rod 14, a furnacebody 17, and upper covers 15 and 16 for sealing the furnace body. At thetime of cooling, the inside of the furnace body is kept in vacuum orreduced-pressure, or inert gas is circulated in the furnace body underthe furnace pressure in a range of from 10⁴ to 10⁵ Pa by a forciblycooling unit 19 and the pumps 10 and 11 for circulating inert gas in thefurnace. The furnace temperature is controlled by a temperaturemonitoring unit 21 to a standby temperature in a range of from 200 to1000°, preferably from 300 to 700° C. The furnace is sealed by the uppercover 16. At the same time when the upper cover 16 is opened and theporous glass preform 1 is put into the furnace, the furnace is sealed bythe upper cover 15 fixed on the upper portion of the seed rod 14. Then,the furnace pressure is reduced to 0.1 to 10 Pa and the furnacetemperature is raised at a rate of from 5to 15° C./min to a temperatureof from 900 to 1350° C., preferably from 1200 to 1300° C. The furnacetemperature is then kept in this temperature range for from 100 to 300minutes, so that gas adsorbed in the porous glass preform is removedsufficiently (first step).

The furnace temperature is further raised at a rate of from 1 to 10° C.per minute to a temperature of from 1250 to 1450° C. (second step), andthen raised to a temperature of from 1460 to 1600° C. and kept at thetemperature for from 5 to 60 minutes (third step). Then, the heating bythe heater is stopped and the inert gas is introduced into the furnacebody. After the pressure in the inside of the furnace body is increasedto from 10⁴ to 10⁵ Pa, the inert gas is circulated in the furnace by theforcibly cooling unit so that the furnace is cooled.

The reference numeral 18 designates a radiation thermometer; 19, aforcibly cooling unit; 20, a traverse mechanism; and 21, a temperaturemonitoring unit. Although the drawing shows the case where both of thefurnace body and the muffle tube are respectively communicated with gassupply units and gas exhaust units, the invention may be applied also toa case where a gas supply unit and a gas exhaust unit are provided foronly one of the furnace body and the furnace core pipe.

Although not shown, valves are provided in the pipings 8, 9, 12 and 13so that evacuation or gas streaming is carried out by switching-over thevalves.

Generally, a porous glass preform synthesized by a vapor-phasesynthesizing method has a structure filled with fine particles of from0.1 to 0.5 μm size. The manner of filling of the fine particles, namelythe bulk density, varies depending on the conditions of vapor-phasesynthesis. For instance, the smaller the particles are and the higherthe temperature is at the time of synthesis, the harder the obtainedporous glass becomes since it has less void holes and hence a greaterbulk density. The porous glass preform for use in the present inventionpreferably has a bulk density not less than 0.6 g/cm³, more preferablyfrom about 0.6 to about 0.8 g/cm³. If the bulk density falls below thisrange, the porous glass preform tends to be broken because it is toosoft. On the other hand, if the bulk density exceeds the above range,the glass preform has too much high hardness and therefore, air bubblesalready incorporated therein are hardly removed and tend to remain.

In the present invention, examination was made as to the manner how theconsolidation state of the porous glass preform changes with respect tothe bulk density of the preform and the degassing time when the porousglass preform was degassed and consolidated in a vacuum consolidationfurnace. Thus, as shown in FIG. 2, the optimum range of the degree ofdegassing of the gas incorporated in the porous glass preform wasobtained, thereby obtaining the relational expression between thedegassing time and the bulk density of the porous glass preform whichfollows:

    Degassing Time (min)=A×Bulk Density (g/cm.sup.3)

wherein A is from 201 to 340.

This is classified into a preferable range and an undesirable range asfollows.

Range a

    Degassing Time>340×Bulk Density

This is not a preferable range in view of the productivity.

Range b

    Degassing Time<201×Bulk Density

This is a range in which products become defective.

Range c

    201×Bulk Density≦Degassing Time≦340×Bulk Density

This is a preferable range in which not only a good preform is obtainedbut also the productivity is good.

The present invention will be described in detail with reference to thefollowing Examples, but the invention should not be construed as beinglimited thereto.

EXAMPLE 1

An intermediate glass preform was elongated to prepare a starting glassrod of 18 mm. A porous glass preform with an outer diameter of 150 mmwas synthesized on the outer circumference of the rod by a VAD method toproduce a composite glass preform with a bulk density of 0.8 g/cm³. Thepreform was consolidated in a vacuum consolidation furnace according tothe configuration of the present invention. The vacuum consolidationfurnace 2 had a muffle tube 3, a heater 4 surrounding the furnace corepipe, ports 8 and 9 for supplying gas into the furnace, and upper covers15 and 16 for sealing the furnace body. At the time of cooling, thefurnace was kept in a vacuum or reduced-pressure atmosphere, or inertgas was circulated in the furnace body under furnace pressure notsmaller than 10⁺⁴ Pa by a forcibly cooling unit 19 and pumps 10 and 11for circulating inert gas in the furnace. In Example 1, the inside ofthe furnace was kept at 400° C. by a temperature monitoring unit 21. Thefurnace was sealed with the upper cover 16. At the same time the uppercover 16 was opened and the preform was put into the furnace, thefurnace was sealed with the upper cover 15 fixed on the upper portion ofthe seed rod 14. Then, the furnace pressure was reduced to 10 Pa by thepumps 10 and 11 and the furnace temperature was raised at the rate of10° C./min. The inside of the furnace was heated to 1300° C. and kept at1300° C. for 240 minutes, so that gas adsorbed in the composite glasspreform was removed sufficiently. Further, the furnace temperature wasraised to a range of from 1500 to 1600° C. at the rate of 3° C./min andkept for 10 minutes. Then, the heating by the heater was stopped andinert gas was introduced into the furnace by the forcibly cooling unit.After the pressure of the inside of the furnace was increased to 10⁺⁵ Paby the inert gas, the inert gas was circulated in the furnace by theforcibly cooling unit so as to cool the furnace. At the point of timewhen the furnace temperature reached 400° C., the circulation of theinert gas in the furnace was stopped and the seed rod 14 was lifted uptogether with the upper cover 15. Then, the furnace was sealed with theupper cover 16. The glass preform taken out was subjected to drawing. Asa result, a good fiber having a loss of 0.335 dB/km at 1.3 μm and 0.195dB/km at 1.55 μm was obtained.

EXAMPLE 2

A porous glass preform prepared by a VAD method and having a diameter of150 mm and a bulk density of 0.7 g/cm³ was degassed and consolidated bythe same apparatus configuration as in Example 1. The inside of thefurnace was kept at 400° C. by the temperature monitoring unit 21 andthe furnace was sealed with the upper cover 16. At the same time theupper cover 16 was opened and the porous glass preform was put into thefurnace, the furnace was sealed by the upper cover 15 fixed on the upperportion of the seed rod 14. Then, the furnace pressure was reduced to 10Pa by the pumps 10 and 11, and the furnace temperature was raised at therate of 10° C./min. The inside of the furnace was heated to 1300° C. andkept at 1300° C. for 210 minutes, so that gas adsorbed in the porousglass preform was removed sufficiently. Further, the furnace temperaturewas raised to a range of from 1500 to 1600° C. at the rate of 3° C./minand then kept for 10 minutes. Then, heating by the heater was stoppedand inert gas was introduced into the furnace. After the pressure of theinside of the furnace was increased to 10⁺⁵ Pa by the inert gas, theinert gas was circulated in the furnace by the forcibly cooling unit soas to cool the furnace. At the point of time when the furnacetemperature reached 400° C., the circulation of the inert gas in thefurnace was stopped and the seed rod 14 was lifted up together with theupper cover 15. As a result, a glass preform having good transparencywas obtained.

Although Examples 1 and 2 have shown the case where the standbytemperature is 400° C., effective standby temperature is from 200 to1000° C., preferably from 300 to 700° C.

Although Examples 1 and 2 have shown the case where a composite glasspreform and a porous glass preform each produced by a VAD method areused, the same effect as in Examples 1 and 2 can be obtained also in thecase where the composite glass preform and porous glass preforms areproduced by other methods such as an OVD method, a sol-gel method, etc.Further, the same effect can be obtained also in the case where thecomposite glass preform and porous glass preforms are produced fromglass particles by molding or pressure-molding.

Although Examples 1 and 2 have shown the case where the degassingtemperature is 1300° C., the degassing temperature is effectively in arange of from 900 to 1350° C., preferably from 1200 to 1300° C.

Comparative Example 1

An intermediate glass preform was elongated to prepare a starting glassrod of 18 mm. A porous glass preform having an outer diameter of 150 mmwas synthesized on the outer circumference of the rod by a VAD method toproduce a composite glass preform having a bulk density of 0.8 g/cm³.The composite preform was consolidated in a vacuum consolidation furnaceaccording to the configuration of the present invention. The vacuumconsolidation furnace 2 had a muffle tube 3, a heater 4 surrounding thefurnace core pipe, ports 8 and 9 for supplying gas into the furnace, andupper covers 15 and 16 for sealing the furnace body. At the time ofcooling, the furnace was kept in a vacuum or reduced-pressureatmosphere, or inert gas was circulated in the furnace body, under thefurnace pressure not smaller than 10⁺⁴ Pa, by a forcibly cooling unit 19and pumps 10 and 11 for circulating inert gas in the furnace body. Inthe Comparative Example 1, the inside of the furnace was kept at 400° C.by a temperature monitoring unit 21, and the furnace was sealed with theupper cover 16. At the same time the upper cover 16 was opened and thecomposite glass preform was put into the furnace, the furnace was sealedwith the upper cover 15 fixed on the upper portion of the seed rod 14.Then, the furnace pressure was reduced to 10 Pa by the pumps 10 and 11,and the furnace temperature was raised at the rate of 10° C./min. Theinside of the furnace was heated to 1300° C. and kept at 1300° C. for 60minutes, so that gas adsorbed in the composite glass preform was removedsufficiently. Further, at 20 minutes after the furnace temperature wasraised to a range of from 1500 to 1600° C. at the rate of 3° C./min,there arose a trouble that the glass preform expanded so much as tostick to the furnace core pipe. Such a trouble is considered to haveoccurred because the duration of 60 minutes in which the temperature of1300° C. was maintained was too short, and as a result the gas thatremained in the glass in the glass preform was caused to expand by theheating temperature for vitrifying the glass preform.

Comparative Example 2

An intermediate glass preform was elongated to prepare a starting glassrod of 18 mm. A porous glass preform having an outer diameter of 150 mmwas synthesized on the outer circumference of the rod by a VAD method toproduce a composite glass preform having a bulk density of 0.5 g/cm³.However, the composite glass preform could not be produced as a productbecause cracking occurred in the surface of the composite glass preformin the process of production.

According to the method of the present invention, the degassing stepwhich is a first step is carried out while the degassing time is changedin accordance with the bulk density of a porous glass preform used.Accordingly, the degassing in preforming can be achieved efficiently, sothat a high-quality transparent glass substance can be obtained.

While the invention has been described in detail and with reference tospecific examples thereof, it will be apparent to one skilled in the artthat various changes and modifications can be made therein withoutdeparting from the spirit and scope thereof.

What is claimed is:
 1. A method for producing a glass preform,comprising the steps ofdegassing a porous glass preform for a degassingtime to thereby remove gas therein wherein the bulk density of saidporous glass preform is not lower than about 0.6 g/cm³ ; temporarilycontracting said porous glass preform at a temperature higher than atemperature in the degassing step and lower than a consolidationtemperature; and consolidating said porous glass preform at theconsolidation temperature;wherein the degassing time is determined bythe expression

    Degassing Time (minute)=A×Bulk Density (g/cm.sup.3),

wherein A is a value from about 201 to
 340. 2. The method for producinga glass preform according to claim 1, wherein said degassing is carriedout at a temperature of from 900 to 1350° C.
 3. The method for producinga glass preform according to claim 1, carried out with an apparatuscomprising:a furnace body; a port opening formed in an upper portion inthe furnace body; a heater disposed in said furnace body; a muffle tubefor isolating said heater and said preform from each other; an uppercover for sealing said port opening after insertion of said preform; aradiation thermometer for monitoring a temperature of said furnace; andan exhaust pump for reducing pressure in said furnace body.
 4. Themethod for producing a glass preform according to claim 1, wherein thesaid bulk density of said porous glass preform is not greater than about0.8 g/cm³.
 5. The method of claim 1, farther comprising the stepsof:forcibly cooling the porous glass preform with an inert gas.
 6. Themethod of claim 5, wherein the inert gas has a pressure between about10⁴ to 10⁵ Pa.
 7. The of method of claim 1, wherein the degassing timeis between about 100 to 300 minutes, based on the bulk density.